1. Field of the Invention
This invention relates generally to the field of chemical vapor deposition for the manufacturing of articles including thin layers of silicon oxide. Particularly, this invention relates to methods and apparatus for controlling the deposition of silicon oxide layers/films upon a series of substrates processed in sequence in the same processing chamber.
2. Brief Description of the Background Art
Chemical vapor deposition (CVD) is a process of forming a film on a substrate, typically, by generating vapors from liquid or solid precursors and delivering those vapors to the (typically heated) surface of a substrate where the vapors react to form a film. Systems for chemical vapor deposition are employed in applications such as semiconductor fabrication, where CVD is employed to form thin films of semiconductors, dielectrics and metal layers. Plasma enhanced chemical vapor deposition (PECVD) is the most common deposition method used to obtain device-quality TEOS-based silicon oxide (SiOx) films. In the current state of the art, for a series of substrates processed sequentially in a single chamber, the TEOS-based silicon oxide deposition rate gradually increases for each subsequent substrate processing, because of changes in the PECVD reactor chamber which occur from substrate to substrate. The lack of a uniform deposition rate during processing of a series of substrates has a negative affect in terms of device performance and/or process yield. To accommodate a potential variation in deposited film thickness, the device design is negatively impacted. Substantial limitations have to be made to the device design.
By way of example, when the substrate is a device such as a thin-film transistor, where the gate insulator film thickness is critical, or a diode, a variation in thickness of a deposited film substantially affects design performance. In such instances, changes in the deposition rate for substrates processed in sequence in a single chamber may quickly breach narrow tolerance limits for the deposited film thickness. One factor which helps improve the consistency of film deposition rate, and in turn thickness repeatability from one processed substrate to the next, is frequent cleaning of the PECVD reaction chamber. During substrate processing, the deposition material attaches to the inner walls and other areas of the reaction chamber, thereby affecting subsequent processing variables. A number of methods for cleaning this deposited material from within a reaction chamber exist, including both wet cleaning and dry cleaning. In wet cleaning, the reaction chamber is opened and the chamber surfaces are cleaned manually. Manual cleaning is very time consuming, negatively affecting substrate processing throughput. Dry cleaning methods are an improvement over wet cleaning methods because dry-cleaning is an in-situ cleaning process that does not require disassembly of the reaction chamber. Typically dry cleaning is used frequently with wet cleaning applied only as necessary.
Numerous examples of process chamber dry cleaning methods exist in the current state of the art, including U.S. Pat. Nos. 5,753,137 and 5,158,644 both assigned to the assignee of the current invention and generally relating to methods for reacting a cleaning species with the contaminant within the reaction chamber to produce a gaseous reaction product which is easily removed from the chamber. Other methods to reduce cleaning frequency requirements include coating the chamber walls with a carbon material that will protect the chamber walls from chemical attack by a reactant processing gas (U.S. Pat. No. 5,085,727); and adding a sacrificial structure to the chamber which prevents polymer build-up within said chamber (U.S. Pat. No. 4,786,359). Frequent remedial cleanings of the substrate processing chamber following a substrate processing will reduce behavioral changes within the processing chamber.
However, even when process chamber surfaces are maintained in a cleaned condition, there are still significant variations in film deposition rate for PECVD deposited films when a series of substrates are processed in sequence in a process chamber. Other process variables affect the film deposition rate during processing of a series of substrates in sequence.
To achieve a reasonable product yield, where the deposited film thickness is held relatively constant, those skilled in the art have resorted to manual adjustment of the deposition time for each substrate processed. This is very labor intensive and subject to error.
It is well known that within a reaction chamber the deposition rate of a source gas is a function of many variables, at least including pressure, gas composition, power, time and temperature. (See e.g., Kim, E J, Gill, W N, Modeling of SiO2 CVD From TEOS/Ozone in a Separate Gas-Injection Reactor, Korean J. Chem. Eng., 15(1), 56-63 (1998) and references therein.). Many methods for controlling deposition rate variability provide materials and methods to control these variables. In general, these methods provide a chemical atmosphere within the reaction chamber that acts to off-set any negative affect caused by deposition material build-up. One example is U.S. Pat. No. 6,723,660 B1 describing a method for reducing the variation of source gas deposition rate to form thin films of a stable thickness by controlling pressure within the reaction chamber during substrate processing. This patent describes how an increase in the temperature of the source gas distribution mechanism (shower head) causes a change in the property of the source gas being deposited onto a substrate. The reference states that the addition of a temperature control device to the reaction chamber to prevent the increase in temperature of the shower head is not feasible, because the complex mechanics of such a temperature control device will negatively affect the delivery of said source gas. Thus, the method of the '660 patent is to counter the increasing showerhead temperature by adjusting the pressure within the processing chamber, thereby stabilizing the deposition rate, to decrease variations in substrate processing. However, this method is limited in that Chemical Vapor Deposition techniques are very sensitive to pressure changes, thus there is only a narrow range of adjustments that can be made in pressure to off-set variability in deposition rate.
Thus there is a need in the art to provide a more consistent reaction chamber environment allowing for a uniform PECVD deposition rate of films during processing of a series of substrates in sequence in a process chamber. In addition there is a need in the art for improved substrate throughput during PECVD deposition of films in the semiconductor industry.